Studies were made concerning the natural mechanisms for transmission, perpetuation, and evolution of type C viruses and virogenes in mammals especially primates. The mechanism of expression and regulation of these genes has been explored with the identification and preliminary characterization of several peptides and factors which effect phenotypic changes typical of malignant behavior in otherwise normal cells. Several new murine and primate endogenous viruses were isolated from mouse species from Southeast Asia and from primates closely related to humans. These murine isolates are different from the usual oncogenic viruses of laboratory mouse in their genetic composition, infectivity, host range, and other properties. The new primate viruses allow the development of specific reagents with which human populations may be examined for the expression of viral genetic material.